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<h1><img src="../../../../boost.png" alt="Boost logo" align=
"middle" width="277" height="86">Boost.Flyweight
Tracking policies reference</h1>

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<h2>Contents</h2>

<ul>
  <li><a href="#preliminary">Preliminary concepts</a></li>
  <li><a href="#tracking">Tracking policies</a></li>
  <li><a href="#tracking_tag_synopsis">Header
    <code>"boost/flyweight/tracking_tag.hpp"</code> synopsis</a>
    <ul>
      <li><a href="#is_tracking">Class template <code>is_tracking</code></a></li>
      <li><a href="#tracking_construct">Class template <code>tracking</code></a></li>
    </ul>    
  </li>
  <li><a href="#refcounted_fwd_synopsis">Header
    <code>"boost/flyweight/refcounted_fwd.hpp"</code> synopsis</a>
  </li>
  <li><a href="#refcounted_synopsis">Header
    <code>"boost/flyweight/refcounted.hpp"</code> synopsis</a>
    <ul>
      <li><a href="#refcounted">Class <code>refcounted</code></a></li>
    </ul>    
  </li>
  <li><a href="#no_tracking_fwd_synopsis">Header
    <code>"boost/flyweight/no_tracking_fwd.hpp"</code> synopsis</a>
  </li>
  <li><a href="#no_tracking_synopsis">Header
    <code>"boost/flyweight/no_tracking.hpp"</code> synopsis</a>
    <ul>
      <li><a href="#no_tracking">Class <code>no_tracking</code></a></li>
    </ul>    
  </li>
</ul>

<h2><a name="preliminary">Preliminary concepts</a></h2>

<p>
A <i>tracking policy helper</i> provides access to some of the functionality
of a <a href="factories.html#factory">factory</a> so as to be used
in the implementation of an associated <a href="#tracking"><code>Tracking Policy</code></a>.
In the following table, <code>TrackingHelper</code> is a tracking policy
helper associated to a <a href="factories.html#factory">factory</a> type 
<code>Factory</code> of elements of type <code>Entry</code>, <code>h</code>
is a value of
<code>Factory::handle_type</code> associated to a <code>Factory</code> <code>f</code>
and <code>check</code> is a value of
a <a href="https://boost.org/sgi/stl/Predicate.html"><code>Predicate</code></a>
type <code>Checker</code> with argument of type <code>Factory::handle_type</code>.
</p>

<p align="center">
<table cellspacing="0">
  <caption><b>Tracking Policy Helper requirements.</b></caption>
<tr>
  <th align="center">expression</th>
  <th align="center">return type</th>
  <th align="center">assertion/note<br>pre/post-condition</th>
</tr>
<tr>
  <td><code>TrackingHelper::entry(h);</code></td>
  <td><code>const Entry&amp;</code></td>
  <td>Returns <code>f.entry(h)</code>.</td>
</tr>
<tr class="odd_tr">
  <td><code>TrackingHelper::erase(h,check);</code></td>
  <td><code>void</code></td>
  <td>If <code>check(h)</code>, invokes <code>f.erase(h)</code>.</td>
</tr>
</table>
</p>

<p>
The execution of <code>TrackingHelper::erase</code> (including the
invocation of <code>check(h)</code>) is done in a
synchronized manner so as to prevent any other thread of execution from
simultaneously accessing the factory's insertion or deletion facilities.
</p>

<h2><a name="tracking">Tracking policies</a></h2>

<p>
A <i>tracking policy</i> defines the strategy to be followed by a
<a href="flyweight.html#flyweight"><code>flyweight</code></a> instantiation when
all the flyweight objects associated to a given value are destroyed.
The tracking policy contributes some type information necessary for the
definition of the <code>flyweight</code> internal
<a href="factories.html#factory">factory</a>.
</p>

<p>
A type <code>Tracking</code> is a tracking policy if:
<ul>
  <li>One of the following conditions is satisfied:
    <ol type="a">
      <li><a href="#is_tracking"><code>is_tracking&lt;Tracking&gt;::type</code></a> is
      <a href="../../../mpl/doc/refmanual/bool.html"><code>boost::mpl::true_</code></a>,</li>
      <li><code>Tracking</code> is of the form
      <a href="#tracking_construct"><code>tracking&lt;Tracking'&gt;</code></a>.</li>
    </ol>
  </li>
  <li>The expression <code>Tracking::entry_type</code> (or 
    <code>Tracking'::entry_type</code> if (b) applies) is an 
    <a href="../../../mpl/doc/refmanual/lambda-expression.html"><code>MPL Lambda
    Expression</code></a> that resolves, when invoked with different types 
    (<code>Value</code>,<code>Key</code>) such that <code>Value</code> is
    <a href="https://boost.org/sgi/stl/Assignable.html"><code>Assignable</code></a>
    and implicitly convertible to <code>const Key&amp;</code>, to an
    <a href="https://boost.org/sgi/stl/Assignable.html"><code>Assignable</code></a>
    type <code>Entry</code> implicitly convertible to both <code>const Value&amp;</code>
    and <code>const Key&amp;</code>.
  </li>
  <li>The expression <code>Tracking::handle_type</code> (or 
    <code>Tracking'::handle_type</code> if (b) applies) is an 
    <a href="../../../mpl/doc/refmanual/lambda-expression.html"><code>MPL Lambda
    Expression</code></a>; this expression, when invoked with types
    (<code>InternalHandle</code>,<a href="#preliminary"><code>TrackingHelper</code></a>),
    with <code>InternalHandle</code> being 
    <a href="https://boost.org/sgi/stl/Assignable.html"><code>Assignable</code></a>
    and providing the nothrow guarantee for copy and assignment,
    resolves to an
    <a href="https://boost.org/sgi/stl/Assignable.html"><code>Assignable</code></a>
    type <code>Handle</code> which also provides the nothrow guarantee for
    copy and assignment and is constructible from and implicitly
    convertible to <code>InternalHandle</code>. 
    <code>TrackingHelper</code> is an incomplete type at the time of
    invocation of <code>Tracking::handle_type</code>.
  </li>
</ul>
<code>Tracking::handle_type</code> is parameterized by a helper that provides
access to some of the functionality of the factory associated to the
tracking policy. This factory's associated entry and handle types are the types
<code>Entry</code> and <code>Handle</code> defined above, respectively.
</p>

<h2><a name="tracking_tag_synopsis">Header
<a href="../../../../boost/flyweight/tracking_tag.hpp"><code>"boost/flyweight/tracking_tag.hpp"</code></a> synopsis</a></h2>

<blockquote><pre>
<span class=keyword>namespace</span> <span class=identifier>boost</span><span class=special>{</span>

<span class=keyword>namespace</span> <span class=identifier>flyweights</span><span class=special>{</span>

<span class=keyword>struct</span> <span class=identifier>tracking_marker</span><span class=special>;</span>

<span class=keyword>template</span><span class=special>&lt;</span><span class=keyword>typename</span> <span class=identifier>T</span><span class=special>&gt;</span>
<span class=keyword>struct</span> <span class=identifier>is_tracking</span>

<span class=keyword>template</span><span class=special>&lt;</span><span class=keyword>typename</span> <span class=identifier>T</span><span class=special>&gt;</span>
<span class=keyword>struct</span> <span class=identifier>tracking</span><span class=special>;</span>

<span class=special>}</span> <span class=comment>// namespace boost::flyweights</span>

<span class=special>}</span> <span class=comment>// namespace boost</span>
</pre></blockquote>

<h3><a name="is_tracking">Class template <code>is_tracking</code></a></h3>

<p>
Unless specialized by the user, <code>is_tracking&lt;T&gt;::type</code> is
<a href="../../../mpl/doc/refmanual/bool.html"><code>boost::mpl::true_</code></a>
if <code>T</code> is derived from <code>tracking_marker</code>, and it is
<a href="../../../mpl/doc/refmanual/bool.html"><code>boost::mpl::false_</code></a>
otherwise.
</p>

<h3><a name="tracking_construct">Class template <code>tracking</code></a></h3>

<p>
<code>tracking&lt;T&gt;</code> is a syntactic construct meant to indicate
that <code>T</code> is a tracking policy without resorting to the
mechanisms provided by the <code>is_tracking</code> class template.
</p>

<h2><a name="refcounted_fwd_synopsis">Header
<a href="../../../../boost/flyweight/refcounted_fwd.hpp"><code>"boost/flyweight/refcounted_fwd.hpp"</code></a> synopsis</a></h2>

<blockquote><pre>
<span class=keyword>namespace</span> <span class=identifier>boost</span><span class=special>{</span>

<span class=keyword>namespace</span> <span class=identifier>flyweights</span><span class=special>{</span>

<span class=keyword>struct</span> <span class=identifier>refcounted</span><span class=special>;</span>

<span class=special>}</span> <span class=comment>// namespace boost::flyweights</span>

<span class=special>}</span> <span class=comment>// namespace boost</span>
</pre></blockquote>

<p>
<code>refcounted_fwd.hpp</code> forward declares the class
<a href="#refcounted"><code>refcounted</code></a>.
</p>

<h2><a name="refcounted_synopsis">Header
<a href="../../../../boost/flyweight/refcounted.hpp"><code>"boost/flyweight/refcounted.hpp"</code></a> synopsis</a></h2>

<h3><a name="refcounted">Class <code>refcounted</code></a></h3>

<p>
<a href="#tracking"><code>Tracking Policy</code></a> providing
<a href="flyweight.html#flyweight"><code>flyweight</code></a>
instantiations with reference counting semantics: when all the flyweight objects
associated to a given value are destroyed, the corresponding entry is
erased from <code>flyweight</code>'s internal
<a href="factories.html#factory">factory</a>.
</p>

<h2><a name="no_tracking_fwd_synopsis">Header
<a href="../../../../boost/flyweight/no_tracking_fwd.hpp"><code>"boost/flyweight/no_tracking_fwd.hpp"</code></a> synopsis</a></h2>

<blockquote><pre>
<span class=keyword>namespace</span> <span class=identifier>boost</span><span class=special>{</span>

<span class=keyword>namespace</span> <span class=identifier>flyweights</span><span class=special>{</span>

<span class=keyword>struct</span> <span class=identifier>no_tracking</span><span class=special>;</span>

<span class=special>}</span> <span class=comment>// namespace boost::flyweights</span>

<span class=special>}</span> <span class=comment>// namespace boost</span>
</pre></blockquote>

<p>
<code>no_tracking_fwd.hpp</code> forward declares the class
<a href="#no_tracking"><code>no_tracking</code></a>.
</p>

<h2><a name="no_tracking_synopsis">Header
<a href="../../../../boost/flyweight/no_tracking.hpp"><code>"boost/flyweight/no_tracking.hpp"</code></a> synopsis</a></h2>

<h3><a name="no_tracking">Class <code>no_tracking</code></a></h3>

<p>
Null <a href="#tracking"><code>Tracking Policy</code></a>: elements inserted
into <code>flyweight</code>'s internal <a href="factories.html#factory">factory</a>
are not erased until program termination.
</p>

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<br>

<p>Revised April 24th 2019</p>

<p>&copy; Copyright 2006-2019 Joaqu&iacute;n M L&oacute;pez Mu&ntilde;oz.
Distributed under the Boost Software 
License, Version 1.0. (See accompanying file <a href="../../../../LICENSE_1_0.txt">
LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">
http://www.boost.org/LICENSE_1_0.txt</a>)
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